In manufacturing semiconductor devices such as LSI and super-LSI or in manufacturing a liquid crystal display panel or the like, a pattern is made by irradiating a ultraviolet light to a semiconductor wafer or a glass plate for liquid crystal, but if a dust particle sticks to a photomask used in this stage, the dust particle obstructs the ultraviolet light or reflects it, causing deformation, short circuit or the like in the pattern being transferred, and such phenomena lead to lowering of the quality of the end products.
Thus, these works are usually performed in a clean room, but, even in a clean room, it is yet difficult to keep the photomask clean all the time. Hence, the exposure light irradiation is conducted only after a surface of the photomask is sheltered by a pellicle as a dust fender. Under such circumstances, foreign particles do not directly adhere to the surface of the photomask, but only onto the pellicle membrane, and thus by setting a photo focus at a pattern on the photomask at the time of lithographing, the foreign particles on the pellicle membrane fail to transfer their shadows onto the photomask.
In general, a pellicle is built up of a pellicle frame, which is made of aluminum, a stainless steel, an engineering plastic or the like, and a transparent pellicle membrane usually made of cellulose nitrate, cellulose acetate, a fluorine-containing polymer or the like which transmit light well; this pellicle membrane is adhered to an upper one of the two annular faces of the pellicle frame. On a lower one of the annular faces of the frame is laid an agglutinant layer made of a polybutene resin, a polyvinyl acetate resin, an acrylic resin, a silicone resin or the like for attaching the pellicle frame to the photomask, and over this agglutinant layer is laid a releasable liner (separator) for protecting the agglutinant layer.
In recent years, owing to the increased refinement of the exposure light pattern, the problem of the deformation of the photomask caused when the pellicle is adhered to it has become more focused. This problem occurs as the photomask and pellicle frame are coupled together via the agglutinant layer, whereby the form of the pellicle frame affects that of the photomask, and thus the pattern described on the surface of the photomask is deformed from the original form; hence, a pellicle is called for that scarcely affects the shape of the photomask when it is attached thereto.
Various countermeasures have been proposed to solve this problem: for example, the mask-boding agglutinant layer is made softer, or the flatness of the pellicle frame is improved. There proposed methods can reduce the influence which the pellicle frame shape imparts to the photomask shape, but the result is not necessarily sufficient. This is on account of the fact that the flatness of the pellicle frame and that of the photomask are not necessarily perfect and, depending on the individual combinations of them the magnitude of the influence imparted by the pellicle frame shape to the photomask shape is irregular.
As is learnt from IP Publications 1 through 3, the conventional countermeasures proposed are basically designed to reduce the rigidity of the pellicle frame so as to enable the pellicle frame to readily fit and copy the shape of the photomask, through a use of a material of low rigidity such as resin or by lowering the height of the pellicle frame or modifying the cross section of the frame bar thereby reducing the cross sectional area.